This invention relates to magnetic powders with modified surfaces for improved dispersibility in binders for the manufacture of magnetic recording media, such as magnetic tapes and discs. More particularly, the invention is concerned with magnetic powders the individual particles of which have a thin coating film formed by plasma polymerization of an organic material containing nitrogen or sulfur.
Ordinary magnetic tapes are made by mixing a ferromagnetic powder of .gamma.-Fe.sub.2 O.sub.3 or other oxide with binder ingredients and then applying the resulting magnetic coating material on a base. The magnetic powders in wide use, e.g., .gamma.-Fe.sub.2 O.sub.3, have hydrophilic groups such as hydroxyl in the surface layers, and the low affinities for binders organic by nature make them hardly dispersible uniformly in the binders. With the ferromagnetic metal powders for metal tapes the uniform dispersion in the binders is even more difficult because, in addition to the --OH and other hydrophilic functional groups on the surface, the powders have higher surface flux densities than .gamma.-Fe.sub.2 O.sub.3 and tend to aggregate objectionably.
For every magnetic recording medium the uniform dispersibility of the magnetic powder in the binder is an important determinant of its performance in relation to the improvements in the packing properties, squareness ratio Br/Bm, etc. In view of this, proposals have already been made to render the magnetic particle surfaces hydrophobic and more affinitive for binders. They include:
(1) Coating inorganic particles with a dispersant. PA1 (2) Coating inorganic particles with a high molecular compound, in the following ways: PA1 (3) Deposition of a certain substance on particular surface, followed by a heat treatment.
(a) Coating with an aqueous high molecular compound. PA2 (b) Coating with an organic solvent type high molecular compound. PA2 (c) Coating by radiation polymerization. PA2 (d) Coating by ultraviolet polymerization. PA2 (e) Coating by mechanochemical polymerization. PA2 (f) Coating by microcapsule technique.
In the method (1), most of the dispersant simply and lightly sticks to the particle surface rather than being adsorbed on the latter in a relatively effective way. Moreover, the amount sticking to the particle surface is so large that, if they are used as such, blooming will occur. Washing the dispersant off the surface would remove the smaller adsorbed portion too, thus losing the coating effect. Furthermore, the operation will lack stability and continuity. For these reasons the method (1) is unpractical. For the coating with a high molecular compound (the method (2)), different approaches (a) to (f) have been proposed. However, none of them have, after all, succeeded in effecting uniform coating without uniformly dispersing the particles in the polymer solution. Ordinarily, such treatments are required because of the poor dispersibility of the particles in organic binders, and nevertheless the treatments themselves have to depend on good dispersibility of the particles for their success. This is, in a sense, a self-contradiction, and therefore the second method including their modifications are unacceptable. Another factor that hinders the commercial acceptance is that in those dispersion systems the inorganic powders and high molecular compounds tend to aggregate together for objectionable size enlargement, imparting undesirable effects upon the dispersion of the particles. The method (3) has shortcomings in that it is not suited for mass production because of a low rate of treatment and that it involves difficulties in controlling the surface conditions of the magnetic particles as desired.
Mechanical dispersion by a pearl mill, three-roll mill, ball mill or the like is also in common use. Even this fails often to ensure adequate dispersion. A fatal drawback is that a prolonged treatment for mechanical dispersion can break the acicularity or other structural features of the magnetic particles. Thus mechanical dispersion has its limitations in use.
The polymeric coating films for the purpose of improving the powder dispersibility should be evenly and thinly formed on the particles. From this viewpoint none of the above-mentioned methods of the prior art are satisfactory.
Under such circumstances, development of a novel polymeric coating method to replace the foregoing has been called for in order to attain improvements in electrical properties as well as in the packing properties through increased dispersibility of magnetic powders and prevent dusting of the powders. After our intensive studies about the method for polymeric coating of magnetic particles with a uniform and thin film in a stabilized operation, it has now been found that the end is best attained by forming a film of a nitrogen- or sulfur-containing organic material by plasma polymerization on magnetic particles. The present invention is based upon this discovery.